The present invention relates to the field of silicon pressure sensors, specifically pressure sensors having a frame or sidewall having an inner side that is substantially orthogonal to a pressure sensor diaphragm and a backside opening having a cross shape.
Conventional pressure sensors include a diaphragm supported by a frame or sidewall. The frame or sidewall is typically made from the same silicon block as the diaphragm, and is formed by etching a backside cavity in the silicon block.
FIG. 1 A is a cross-section of a conventional pressure sensor 100 that may be improved by incorporation of embodiments of the present invention. This pressure sensor includes a diaphragm 110 supported by a frame 120. These sensors are conventionally fabricated by selectively etching a <100> silicon wafer using KOH. Specifically, a silicon wafer or block is etched most of the way through resulting in a backside cavity 130 having a backside opening 135.
The backside cavity 130 is defined by the backside opening 135 and an inner sidewall of the frame 120, which has a slope of 54.7 degrees as dictated by the <111> crystal plane. Accordingly, for each micron of depth 125 of the cavity 130, the backside opening 135 is increased by 0.708 microns in width. For example, a 10 microns thick, thousand micron wide diaphragm in a 410 microns thick silicon wafer requires an additional length 137 equal to 283 microns for a total backside opening 135 of 1566 microns. In practice, the actual die size is even larger. This is because the sidewalls require a certain width since these die are conventionally bonded to glass or other type substrate before the die in the wafer are individualized. This additional length 127 in one example is 255 microns. In that case, using the above numbers, the actual die length becomes 2076 microns. In another example, the diaphragm length may be reduced to 250 microns. In that case, the die length is reduced to 1326 microns.
As can be seen from FIG. 1A, a large part of the length of these sensors is due to the sloped edges of the frame 120. Accordingly, if the edge of the frame 120 is made substantially vertical, the overall die length is reduced. For example, if the diaphragm length is maintained at 250 microns and the sidewall width 127 is held at 255 microns, the die length is decreased from 1326 to 760 microns. Given a 0.1 mm line width required for dicing the wafer into individual die, this length reduction means 2.75 times the number of sensors may be fabricated on a wafer. Thus, what is needed are methods and apparatus for extremely low cost pressure sensors having a frame or sidewall that is substantially orthogonal to the sensor diaphragm. It is also desirable that the sensitivity of the pressure sensor be variable by changing a limited number of masks and processing steps.